When the page first loads it looks fine for a split second, and then the right margin becomes so wide it pushes main content into a sliver.  

Steps to Reproduce

Go to any admin page at mobile breakpoint.

Operating System

iOS

Browser

Google Chrome

Related Entity

I notice it on iOS using my phone, but have also been able to replicate on MacOS/Chrome by resizing the browser to mobile breakpoint.     By the looks of it, has somthing to do with the CSS reference highlighted in the second screen grab (.fixed-sidebar .mini-navbar #page-wrapper in global)   I have purged cache too, as thought it might be old CSS file or something, but that didn't help either.